[qemu-xen master] xen-bus/block: explicitly assign event channels to an AioContext

[patchbot@xxxxxxx]

commit 63cb54f1fe78f2c2365e46d4dccbf30831077049
Author:     Paul Durrant <pdurrant@xxxxxxxxxx>
AuthorDate: Mon Dec 16 14:34:51 2019 +0000
Commit:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
CommitDate: Fri Feb 28 14:52:52 2020 +0000

    xen-bus/block: explicitly assign event channels to an AioContext
    
    It is not safe to close an event channel from the QEMU main thread when
    that channel's poller is running in IOThread context.
    
    This patch adds a new xen_device_set_event_channel_context() function
    to explicitly assign the channel AioContext, and modifies
    xen_device_bind_event_channel() to initially assign the channel's poller
    to the QEMU main thread context. The code in xen-block's dataplane is
    then modified to assign the channel to IOThread context during
    xen_block_dataplane_start() and de-assign it during in
    xen_block_dataplane_stop(), such that the channel is always assigned
    back to main thread context before it is closed. aio_set_fd_handler()
    already deals with all the necessary synchronization when moving an fd
    between AioContext-s so no extra code is needed to manage this.
    
    Reported-by: Julien Grall <jgrall@xxxxxxxxxx>
    Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
    Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
    Message-Id: <20191216143451.19024-1-pdurrant@xxxxxxxxxx>
    Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
    (cherry picked from commit 32d0b7be68216f130dc962533c6fc2f9ad848a8b)
---
 hw/block/dataplane/xen-block.c | 20 ++++++++++++++++++--
 hw/xen/xen-bus.c               | 27 +++++++++++++++++++++++----
 include/hw/xen/xen-bus.h       |  5 ++++-
 3 files changed, 45 insertions(+), 7 deletions(-)

diff --git a/hw/block/dataplane/xen-block.c b/hw/block/dataplane/xen-block.c
index 0f200c5fb0..18ba4ac897 100644
--- a/hw/block/dataplane/xen-block.c
+++ b/hw/block/dataplane/xen-block.c
@@ -685,12 +685,24 @@ void xen_block_dataplane_stop(XenBlockDataPlane 
*dataplane)
         return;
     }
 
+    xendev = dataplane->xendev;
+
     aio_context_acquire(dataplane->ctx);
+    if (dataplane->event_channel) {
+        /* Only reason for failure is a NULL channel */
+        xen_device_set_event_channel_context(xendev, dataplane->event_channel,
+                                             qemu_get_aio_context(),
+                                             &error_abort);
+    }
     /* Xen doesn't have multiple users for nodes, so this can't fail */
     blk_set_aio_context(dataplane->blk, qemu_get_aio_context(), &error_abort);
     aio_context_release(dataplane->ctx);
 
-    xendev = dataplane->xendev;
+    /*
+     * Now that the context has been moved onto the main thread, cancel
+     * further processing.
+     */
+    qemu_bh_cancel(dataplane->bh);
 
     if (dataplane->event_channel) {
         Error *local_err = NULL;
@@ -807,7 +819,7 @@ void xen_block_dataplane_start(XenBlockDataPlane *dataplane,
     }
 
     dataplane->event_channel =
-        xen_device_bind_event_channel(xendev, dataplane->ctx, event_channel,
+        xen_device_bind_event_channel(xendev, event_channel,
                                       xen_block_dataplane_event, dataplane,
                                       &local_err);
     if (local_err) {
@@ -818,7 +830,11 @@ void xen_block_dataplane_start(XenBlockDataPlane 
*dataplane,
     aio_context_acquire(dataplane->ctx);
     /* If other users keep the BlockBackend in the iothread, that's ok */
     blk_set_aio_context(dataplane->blk, dataplane->ctx, NULL);
+    /* Only reason for failure is a NULL channel */
+    xen_device_set_event_channel_context(xendev, dataplane->event_channel,
+                                         dataplane->ctx, &error_abort);
     aio_context_release(dataplane->ctx);
+
     return;
 
 stop:
diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index 63469e0cf9..cf79c5e3a8 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -1089,8 +1089,26 @@ static void xen_device_event(void *opaque)
     }
 }
 
+void xen_device_set_event_channel_context(XenDevice *xendev,
+                                          XenEventChannel *channel,
+                                          AioContext *ctx,
+                                          Error **errp)
+{
+    if (!channel) {
+        error_setg(errp, "bad channel");
+        return;
+    }
+
+    if (channel->ctx)
+        aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
+                           NULL, NULL, NULL, NULL);
+
+    channel->ctx = ctx;
+    aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
+                       xen_device_event, NULL, xen_device_poll, channel);
+}
+
 XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
-                                               AioContext *ctx,
                                                unsigned int port,
                                                XenEventHandler handler,
                                                void *opaque, Error **errp)
@@ -1116,9 +1134,10 @@ XenEventChannel *xen_device_bind_event_channel(XenDevice 
*xendev,
     channel->handler = handler;
     channel->opaque = opaque;
 
-    channel->ctx = ctx;
-    aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
-                       xen_device_event, NULL, xen_device_poll, channel);
+    /* Only reason for failure is a NULL channel */
+    xen_device_set_event_channel_context(xendev, channel,
+                                         qemu_get_aio_context(),
+                                         &error_abort);
 
     QLIST_INSERT_HEAD(&xendev->event_channels, channel, list);
 
diff --git a/include/hw/xen/xen-bus.h b/include/hw/xen/xen-bus.h
index 3d5532258d..c18c1372af 100644
--- a/include/hw/xen/xen-bus.h
+++ b/include/hw/xen/xen-bus.h
@@ -128,10 +128,13 @@ void xen_device_copy_grant_refs(XenDevice *xendev, bool 
to_domain,
 typedef bool (*XenEventHandler)(void *opaque);
 
 XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
-                                               AioContext *ctx,
                                                unsigned int port,
                                                XenEventHandler handler,
                                                void *opaque, Error **errp);
+void xen_device_set_event_channel_context(XenDevice *xendev,
+                                          XenEventChannel *channel,
+                                          AioContext *ctx,
+                                          Error **errp);
 void xen_device_notify_event_channel(XenDevice *xendev,
                                      XenEventChannel *channel,
                                      Error **errp);
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#master